This invention pertains to, inter alia, charged-particle-beam (CPB) projection-optical systems for use in xe2x80x9cmappingxe2x80x9d CPB (e.g., electron beam or ion beam) microscopes, to methods for adjusting such projection-optical systems, and to use of such microscopes for observing and inspecting surfaces of objects.
Charged-particle-beam (xe2x80x9cCPBxe2x80x9d, e.g., electron beam or ion beam) microscopes are in routine use for observing and inspecting intricate and highly integrated semiconductor circuits and the like as formed on suitable substrates. Such CPB microscopes include scanning electron microscopes (SEMs) and xe2x80x9cmapping electron microscopesxe2x80x9d. Whereas an SEM performs illumination and imaging from one point to another point on a specimen, a mapping electron microscope performs illumination and imaging from one surface to another surface of the specimen. Much research and development has been directed in recent years to improving the CPB mapping projection-optical systems used in mapping electron microscopes.
The structure of a conventional mapping electron microscope is summarized below, with reference to FIG. 1. A primary electron beam (also termed an xe2x80x9cirradiation electron beamxe2x80x9d) PB is emitted by an electron gun 21. The primary electron beam PB passes through an irradiation lens system 22 and enters a Wien filter 25. The Wien filter 25 typically comprises a magnetic pole 26 and an electrical pole 27. The Wien filter 25 bends the trajectory of the primary electron beam PB. After passing through the Wien filter 25, the primary electron beam PB passes through an aligner 30 and through an objective lens system 24 so as to be incident on the surface of a specimen 23. The irradiation lens system 22, Wien filter 25, aligner 30, and objective lens system 24 are collectively termed the xe2x80x9cirradiation optical systemxe2x80x9d or xe2x80x9cprimary optical system.xe2x80x9d
Impingement of the primary electron beam PB on the surface of the specimen 23 generates relatively high-energy electrons that are reflected from the surface of the specimen 23 and relatively low-energy secondary electrons that are emitted from the surface of the specimen 23. The secondary electrons are normally used for imaging. The secondary electrons (formed into an xe2x80x9cobservation electron beamxe2x80x9d or xe2x80x9csecondary electron beamxe2x80x9d OB) returns through the objective lens system 24 and the aligner 30 and re-entered the Wien filter 25. Rather than experiencing trajectory bending by the Wien filter 25, the observation electron beam OB passes straight through the Wien filter 25. The observation electron beam OB then passes through an imaging lens system 28 and enters a detector 29. Observations of the specimen 23 are based on information in the observation electron beam OB as detected by the detectors 29. The objective lens system 24, aligner 30, Wien filter 25, and imaging lens system 28 collectively comprise a xe2x80x9cmapping optical systemxe2x80x9d or xe2x80x9csecondary optical system.xe2x80x9d
The Wien filter 25 is an electromagnetic prism also termed an xe2x80x9cExc3x97Bxe2x80x9d (xe2x80x9cE cross Bxe2x80x9d). By imposing Wien""s condition on the primary electron beam PB, the Wien filter 25 imparts a desired deflection to the trajectory of the primary electron beam PB, while not deflecting the trajectory of the secondary electron beam OB. Upon passing through the Wien filter 25, the primary electron beam PB can have, e.g., a linear, rectangular, circular, or elliptical transverse (sectional) profile.
It is necessary to be able to accurately adjust various components of the CPB mapping projection-optical system (e.g., align the illumination field of the primary optical system with the observation field of the secondary optical system) before use in order to accurately observe and inspect the surface of the specimen 23. To such end, it would be advantageous to be able to independently adjust (e.g., alignment with optical axis, aberration correction) the primary optical system, the secondary optical system, and the Wien filter 25 (e.g., by adjusting respective voltages (or currents) applied to components in the primary optical system, the secondary optical system, and the cathode lens, and by adjusting the electromagnetic field generated by the Wien filter 25). Conventional adjustment methods require excessive time and effort to perform.
It also would be advantageous to be able to determine positional coordinates of the specimen being observed or inspected using a CPB mapping microscope. According to one conventional scheme for making such a determination, an off-axis light-optical system (i.e., an optical system for light) is used in conjunction with the CPB optical system. In such a scheme, the specimen is mounted on a stage provided with fiducial marks (e.g., a pattern of lines and spaces). Unfortunately, however, conventional practice has revealed much difficulty in detecting such marks using both a light-optical system and a CPB optical system. Difficulty is also conventionally encountered in detecting fiducial marks configured as a grooved pattern (e.g., scribe lines), which readily can be detected using an optical microscope but not by a CPB optical system.
In other words, marks that can be detected readily using light are usually not detectable using a charged particle beam. This makes it difficult to select a fiducial mark that is optimal for use with both a CPB optical system and an off-axis light-optical system.
According to another conventional method for evaluating optical performance (e.g., resolution and aberration) of a CPB mapping microscope an xe2x80x9cevaluation chartxe2x80x9d is placed at the position of the specimen 23 in FIG. 1. The evaluation chart is typically a pattern comprising ultra-fine features defined by deposition or microlithography. The evaluation chart is irradiated using the primary electron beam PB, and an image is produced from the observation beam OB generated therefrom.
Unfortunately, whenever optical performance is evaluated using an evaluation chart in such a manner, the optical axis of the irradiation optical system and the optical axis of the mapping optical system must be simultaneously adjusted by making simultaneous adjustments to the Wien filter and the aligner. This requires that the evaluation chart be uniformly illuminated with the primary electron beam PB in order to check the optical performance of the mapping electron microscope. The Wien""s condition is found while continuously changing the electromagnetic pole induction parameters in the Wien filter 25 so that the trajectory of the secondary electron beam is not deflected. Changing the electromagnetic pole induction parameters in such a manner causes a simultaneous change in the uniformity of illumination by the primary electron beam. Consequently, it is necessary to continually readjust the optical axis of the illumination optical system. In addition, whenever the secondary electron beam is deflected by the aligner and axially aligned with the objective lens system, the primary electron beam is simultaneously deflected, thereby changing the uniform illumination and making it necessary again to readjust the optical axis of the illumination optical system. Thus, such conventional evaluations of optical performance are extremely complex to perform.
The kinetic-energy distribution of electrons in the secondary electron beam emitted from the specimen is very sensitively affected by the type and shape of the specimen and the irradiation angle of the secondary electron beam. This instability of the kinetic-energy distribution of the secondary electron beam adds even more complexity to conventional evaluations of the optical performance of the mapping electron microscope, and makes it impossible to determine, e.g., the magnitude of chromatic aberration.
The shortcomings of the prior art noted above are addressed by the present invention which provides, inter alia, apparatus for charged-particle-beam (CPB) projection-optical systems, and methods for adjusting such systems, allowing rapid and accurate adjustments, even by a relatively unskilled operator.
According to one aspect of the invention, charged-particle-beam mapping projection-optical systems are provided. Representative embodiments of such systems comprise an irradiation optical system, an Exc3x97B beam separator (i.e., Wien filter or xe2x80x9cExc3x97Bxe2x80x9d), an objective optical system, an imaging optical system, and an adjustment-beam source. The irradiation-optical system directs an irradiation charged particle beam along a first axis from an irradiation-beam source. The Exc3x97B beam separator is configured and situated to receive the irradiation beam from the irradiation optical system and to direct the irradiation beam downstream of the Exc3x97B beam separator. The objective optical system is configured and situated to receive the irradiation beam from the Exc3x97B beam separator, direct the irradiation beam to be incident on a surface of a specimen located at a position downstream of the objective optical system, receive an observation charged particle beam generated by impingement of the irradiation beam on the specimen surface, and direct the observation beam to the Exc3x97B beam separator. The Exc3x97B beam separator causes the observation beam to propagate along a second axis having a direction different than the first axis. The imaging optical system is configured and situated to receive the observation beam from the Exc3x97B beam separator and to direct the observation beam from the Exc3x97B beam separator to a detector. The adjustment-beam source is configured to emit an adjustment charged particle beam, and is situatable at the specimen position so as to direct the adjustment beam, in place of the observation beam, through the objective and imaging optical systems to the detector.
The adjustment beam produced by the adjustment-beam source has an emission profile at the specimen position. The emission profile desirably corresponds to at least one of a dot, a line, a plane, a cross, or an L-shaped profile.
The adjustment beam can be any of various charged particle beams, such as an electron beam. The adjustment-beam source desirably produces the adjustment beam having a kinetic energy equal to a kinetic energy of the observation beam as generated at the specimen surface. An exemplary adjustment-beam source is a cold cathode. To provide an acceleration of the adjustment beam as it propagates to the detector, an electrode can be situated object-wise of the objective optical system so as to generate a potential relative to the adjustment-beam source sufficient to accelerate the adjustment beam as the adjustment beam propagates to the detector.
According to another aspect of the invention, methods are provided for operating a charged-particle-beam mapping projection microscope. In representative embodiments of such methods, an irradiation charged particle beam is directed along a first axis from an irradiation-beam source through an irradiation optical system to an Exc3x97B beam separator, then passed through the Exc3x97B beam separator and through an objective optical system so as to cause the irradiation beam to impinge on a surface of a specimen at an object surface. Such impingement generates, from the impingement, an observation charged particle beam propagating from the specimen toward the objective optical system. The observation beam is passed through the objective optical system and the Exc3x97B beam separator along a second axis having a different direction than the first axis, and then through an imaging optical system to a detector. The subject methods comprise a process for adjusting the objective optical system and imaging optical system. In such an adjustment process, the specimen (situated at the object surface) is replaced with an adjustment-beam source that emits an adjustment charged particle beam. While passing the adjustment beam through the objective optical system, the Exc3x97B beam separator, and the imaging optical system, electrical power is applied only to the objective optical system. Meanwhile, one or more of an axial alignment and an aberration characteristic of the objective optical system is determined. If desired or required, the one or more of an axial alignment and an aberration characteristic of the objective optical system can be adjusted based on the determination.
Electrical power can be applied to the imaging optical system as well as the objective optical system, during which one or more of an axial alignment and an aberration characteristic of the imaging optical system is determined. If desired or required, the one or more of an axial alignment and an aberration characteristic of the imaging optical system can be adjusted based on the determination.
According to another aspect of the invention, CPB mapping projection-optical systems are provided. Representative embodiments of such systems comprise an irradiation optical system, an Exc3x97B beam separator, an objective optical system, an imaging optical system, an alignment-beam source, and an alignment optical system. The irradiation optical system directs an irradiation charged particle beam along a first axis from an irradiation-beam source. The Exc3x97B beam separator is configured and situated so as to receive the irradiation beam from the irradiation optical system and to direct the irradiation beam downstream of the Exc3x97B beam separator. The objective optical system is configured and situated to receive the irradiation beam from the Exc3x97B beam separator, direct the irradiation beam to be incident on a specimen surface located at an object-surface plane downstream of the objective optical system, receive an observation charged particle beam generated by impingement of the irradiation beam on the specimen surface, and direct the observation beam to the Exc3x97B beam separator, wherein the Exc3x97B beam separator causes the observation beam to propagate along a second axis having a direction different than the first axis. The imaging optical system is configured and situated and receive the observation beam from the Exc3x97B beam separator and to direct the observation beam from the Exc3x97B beam separator to a first detector. The alignment-beam source is configured to emit an alignment beam with respect to the object-surface so as to cause the alignment beam to acquire data regarding an alignment characteristic of the object surface. The alignment optical system is situated off-axis from the objective and imaging optical systems and is configured to direct the alignment beam from the object surface to a second detector that detects the data.
The alignment-beam source can be situated at and movable within the object-surface plane. For example, the alignment-beam source can be defined on a fiducial plate, and the fiducial plate can comprise a fiducial mark. In another embodiment, the alignment-beam source is situated remotely from the object-surface plane and is directed by a lens to the object surface. In the latter instance, a fiducial mark can be situated on the object-surface plane. The fiducial mark is desirably configured to be optimal for the irradiation optical system, the objective optical system, the imaging optical system, and the off-axis optical system.
By way of example, the alignment beam can be a beam of light or a charged particle beam. In the latter instance, the alignment beam can be an electron beam, wherein the off-axis optical system is a scanning electron microscope, and the alignment-beam source desirably has an emission profile (at the object surface) that is at least one of a dot, a line, a cross, or an L-shaped profile. As a charged particle beam, the alignment beam desirably has a kinetic energy equal to the kinetic energy of the observation beam. To produce a CPB alignment beam, the alignment-beam source can be a cold cathode.
In addition, an electrical potential can be imposed between the alignment-beam source and an object-wise surface of the objective optical system. In such an instance, the potential causes an acceleration of the alignment beam as the alignment beam propagates through the objective optical system.
According to another aspect of the invention, methods are provided for operating a charged-particle-beam mapping projection microscope. In such methods, an irradiation charged particle beam is directed along a first axis from an irradiation-beam source through an irradiation optical system to an Exc3x97B beam separator, then passed through the Exc3x97B beam separator and through an objective optical system so as to cause the irradiation beam to impinge on a surface of a specimen at an object surface. Such impingement generates an observation charged particle beam propagating from the specimen toward the objective optical system. The observation beam is passed through the objective optical system and the Exc3x97B beam separator along a second axis having a different direction than the first axis, and then through an imaging optical system to a detector. In such methods, a process is provided for adjusting the objective optical system and the imaging optical system. A representative embodiment of such a process comprises placing an adjustment-beam source at the object surface (the adjustment-beam source being operable to emit an adjustment charged particle beam). An electrical potential and electrical current applied to the Exc3x97B beam separator are adjusted so as to align an image formed on the detector by the adjustment-beam source when an electrical potential and electrical current are not applied to the Exc3x97B beam separator with an image formed on the detector by the adjustment-beam source when an electrical potential and electrical current are applied to the Exc3x97B beam separator. The imaging optical system can comprise a stigmator that corrects aberration in the image formed on the detector. Also, electrical energy applied to at least one of the objective optical system and the imaging optical system can be adjusted while adjusting the electrical energy applied to the detector.
By way of example, the adjustment beam can be an electron beam. In such an instance, the adjustment beam desirably has a kinetic energy equal to the kinetic energy of the observation beam.
The process can further comprise providing a potential difference between the adjustment-beam source and a specimen-wise surface of the objective optical system, wherein the potential difference serves to accelerate the adjustment beam.
According to another aspect of the invention, CPB mapping projection-optical systems are provided, that comprise an irradiation optical system, an Exc3x97B beam separator, an objective optical system, an imaging optical system, and an adjustment-beam source. The irradiation optical system directs an irradiation charged particle beam along a first axis from an irradiation-beam source. The Exc3x97B beam separator is configured and situated to receive the irradiation beam from the irradiation optical system and to direct the irradiation beam downstream of the Exc3x97B beam separator. The objective optical system is configured and situated to receive the irradiation beam from the Exc3x97B beam separator, direct the irradiation beam to be incident on a specimen surface located at an objective-surface plane downstream of the objective optical system, receive an observation charged particle beam generated by impingement of the irradiation beam on the specimen surface, and direct the observation beam to the Exc3x97B beam separator, wherein the Exc3x97B beam separator causes the observation beam to propagate along a second axis having a direction different than the first axis; the imaging optical system is configured and situated to receive the observation beam from the Exc3x97B beam separator and to direct the observation beam from the Exc3x97B beam separator to a first detector. The adjustment-beam source is configured to emit an adjustment beam with respect to the object-surface so as to cause the adjustment beam to acquire data regarding a position of the object surface. Desirably, the Exc3x97B beam separator is connected to a variable-power supply to permit an electrical potential and electrical current applied to the Exc3x97B beam separator to be adjusted as required such that an image formed on the detector by the adjustment beam when the electrical potential and electrical current are not applied to the Exc3x97B beam separator is aligned with an image formed on the detector by the adjustment beam when the electrical potential and electrical current are applied to the Exc3x97B beam separator.
Preferably, the imaging optical system includes stigmators that correct aberration in the image formed on the detector. In the method, the voltage (or current) applied to at least one of the objective optical system and the imaging optical system is adjusted while adjusting the voltage applied to the detector.
According to another aspect of the invention, CPB mapping projection-optical systems are provided. Representative embodiments of such a system comprise an irradiation optical system, an Exc3x97B beam separator, an objective optical system, an imaging optical system, an adjustment-beam source, and an xe2x80x9cevaluation chart.xe2x80x9d The irradiation optical system directs an irradiation charged particle beam along a first axis from an irradiation-beam source. The Exc3x97B beam separator is configured and situated to receive the irradiation beam from the irradiation optical system and to direct the irradiation beam downstream of the Exc3x97B beam separator. The objective optical system is configured and situated to receive the irradiation beam from the Exc3x97B beam separator, direct the irradiation beam to be incident on a specimen surface located at an object-surface plane downstream of the objective optical system, receive an observation charged particle beam generated by impingement of the irradiation beam on the specimen surface, and direct the observation beam to the Exc3x97B beam separator. The Exc3x97B beam separator causes the observation beam to propagate along a second axis having a direction different than the first axis. The imaging optical system is configured and situated to receive the observation beam from the Exc3x97B beam separator and to direct the observation beam from the Exc3x97B beam separator to a first detector. The adjustment-beam source is configured to emit an adjustment beam with respect to the object-surface so as to cause the adjustment beam to acquire data regarding a position of the object surface. The evaluation chart is configured so as to be insertable at the object-surface plane. The evaluation chart spontaneously emits an evaluation electron beam for evaluating an optical-performance characteristic of the imaging optical system. The evaluation electron beam desirably has a kinetic energy that is equal to the kinetic energy of the observation beam. Also, the evaluation electron beam desirably has an emission profile such as a dot-shaped profile, a line-shaped profile, or a planar profile.
The evaluation chart can comprise a hot-electron emitter. The evaluation chart is desirably disposed so that it can be inserted and removed at the position of the specimen surface. Such an evaluation chart spontaneously emits an evaluation electron beam for inspecting the optical performance of the mapping optical system. The kinetic energy of the evaluation beam is desirably equal to the kinetic energy of the observing beam.
It is also preferable for the emission profile of the evaluation beam to have any one of a dot shape, a line shape, or a plane shape.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.